The hydroformylation of allyl alcohol is known and is utilized industrially (see e.g. U.S. Pat. No. 4,064,145; U.S. Pat. No. 4,215,077; U.S. Pat. No. 4,238,419; U.S. Pat. No. 4,678,857; U.S. Pat. No. 5,290,743). Allyl alcohol is reacted in these processes with CO/H2 gas mixtures, giving 4-hydroxybutyraldehyde (HBA). Following distillative removal of undesired by-products, HBA is hydrogenated in a known manner to give 1,4-butanediol (BDO).
Recently, rhodium complexes together with diphosphine ligands such as DIOP (2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis-[bis(3,5-dimethylphenyl)phosphino]-butane) have been employed for hydroformylation reactions.
A general disadvantage of this mode of production is the formation of undesired by-products. In particular, as well as the desired linear product, the isomeric branched product 3-hydroxy-2-methylpropionaldehyde (HMPA) and other C3 by-products such as n-propanol and propionaldehyde are formed. This adversely affects the economic viability of the process.
The present invention provides phosphine ligands which are based on a cyclobutane ligand containing at least two trans-coordinated (3,5-dialkyl-phenyl)phosphinomethyl groups. In a further aspect, the present invention provides catalyst systems which are formed from a rhodium complex and said cyclobutane ligand comprising at least two trans-coordinated (3,5-dialkylphenyl)-phosphinomethyl groups.
The ligands and catalyst systems of the present invention allow more favorable HBA:HMPA-proportions to be achieved in the hydroformylation of allyl alcohole, and thus provide improved selectivity and high reaction yields.